Method for preparing a ceramic gas-turbine nozzle with cooling fine holes

ABSTRACT

To improve the precision of the cooling fine holes of a large-sized and complex-shaped ceramic gas-turbine nozzle with fine cooling holes, a ceramic gas-turbine nozzle with fine cooling holes has a profile degree of 1.0% or less relative to the blade length, a roundness of the fine cooling holes of 0.3 mm or less, and a straightness of the fine cooling holes of 1% or less relative to the blade length. This nozzle is prepared by a method in which two compacts of shroud portions and at least one compact of a blade portion and separately molded are joined by isotropic press molding to be calcined, and the predetermined positions of the resulting calcined compact are subjected to supersonic processing to bore cooling fine holes. The resulting compact is then fired.

This is a Division of application Ser. No. 08/038,808, filed Mar. 29,1993, now U.S. Pat. No. 5,342,166.

BACKGROUND OF THE INVENTION

The present invention relates to a ceramic gas-turbine nozzle with finecooling holes and a method for preparing the same. The ceramicgas-turbine nozzle with fine cooling holes of the present invention canbe preferably used as, for example, a dynamo gas turbine nozzle.

Recently, dynamo gas turbines have been intensively developed in orderto achieve energy-savings; decreased environmental pollution andusability of various fuels. Particularly, the research for developmentof a the gas turbine with large heat efficiency or high energy savingsis vigorously performed, and causes the related researchers to focustheir attention on a problem that the inlet gas temperature (TIT) of theturbine is elevated to improve the efficiency of heat engines. In fact,a metal gas turbine using a heat-resistant alloy has a maximum inlet gastemperature of 1350° C.

An attempt has been made to further elevate the turbine inlet gastemperature by applying ceramics which are superior excellent in heatresistance than the heat-resistant alloy to parts such as gas turbineblades, and so gas turbines using a ceramic have already been developedwhich have almost the same TIT as those using a heat-resistant alloy.Thus, the current development of gas turbines aims at preparing thosewith a TIT of nearly 1500° C.

When a ceramic component, however, is used for a gas turbine nozzlehaving a TIT of more than 1500° C., a local area having a TIT of morethan 1600° C. may be present. Under such circumstances, even if adesired heat-resistant ceramic is obtained, there arise problems ofdecrease in mechanical strength, or potential influence of erosion orcorrosion, reduced reliability, shortened service life and the like.

The present inventors have now developed a ceramic component with finecooling holes in which a refrigerant carrier flows through the fineholes bored at predetermined positions in the ceramic component toimprove the heat resistance of the ceramic component; Japanese PatentLaid-Open No. 4-219205 (Japanese Patent Application No. 3-87581). Sincethe surface of this ceramic component is cooled by a refrigerantcarrier, even if the TIT is elevated to 1500° C. or more, the surface ofthe ceramic gas-turbine nozzle is kept at lower temperatures, e.g.,about 1100° C.; thus there are no problems about decreased reliabilityand mechanical strength and the like under a higher temperature. Thelower temperatures hardly have adverse effects on the heat efficiency ofthese gas turbines.

However, the publicly known methods have disadvantages if the ceramiccomponent is provided with the fine cooling holes. Examples of theconventional methods by which the fine holes are bored include a methodwhere piano wire or the like is placed in the mold at the time ofinjection molding, or grinding processing or supersonic processing aftersintering.

For example, when injection molding is performed, there are problemsthat the piano wire is cut off because of the increased pressure ofinjection molding, and the position and the like of fine holes arelimited depending on the mold structure. There are also problems thatthe fine holes having smaller diameters may make it impossible tosuccessfully release the fine hole portions from the mold.

Similarly, only straight fine holes can be made by the grinding processor supersonic process after sintering, with the limitation of theirdeepness. These processings make the beveling processing impossible.Particularly, when the fine hole is to be made so as to have a diameterof about 0.5 mm, its processing time will be greatly prolonged and itsmass production will be poorly attained.

Alternatively, when a ceramic is applied to the gas-turbine nozzleparts, there arises a problem characteristic of the gas turbine nozzle.Having large size, the gas turbine nozzle provides complex shapes, sothat the conventional methods for preparing ceramic parts have greatlimitations of parts' shapes and dimensions, which are ascribed to theirmolding shape, moldability, sinterability, processability and the like.For example, in the case of a large-sized gas-turbine nozzle, the largerbody makes it difficult to be molded by even any method such asinjection molding and press molding. Particularly, the present method ofinjection molding does not provide the resulting moldings with uniformdensity, and so their large deformation caused after sintering cannotmold the compacts or parts with close shape-precision.

Examples of gas turbines having more complex shapes include a gasturbine in which plural blades are put between the inside shroud portionand the outside shroud portion to align linearly or in the form of anarc. There arises a great problem that it is difficult to mold a gasturbine nozzle having a complex shape, which causes undercuts. Thus,when forming a gas turbine in such a shape which will cause undercuts,it is clearly impossible to prepare compacts using the ordinary mold.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a ceramicgas-turbine nozzle with fine cooling holes which has a large-sized,complex shape (their dimensions, structure and the like having beenpreviously limited because ceramics were made into large-sized parts)and has a more improved precision than the earlier one; and a method forpreparing the same.

According to another aspect of the present invention, there is provideda method for preparing a ceramic gas-turbine nozzle with fine coolingholes though it is difficult to process fine holes by the conventionalmethod.

In order to achieve the above objects, according to one aspect of thepresent invention, there is provided a ceramic gas-turbine nozzlecomprising at least one blade portion having a hollow cavity in theinterior and being provided with fine cooling holes, and shroud portionsintegrally joined with said blade portion, and at least one of saidshroud portions being provided with a throughhole, said ceramicgas-turbine nozzle having a profile degree of 1.0% or less relative tothe blade length, said fine cooling holes having a roundness of 0.3 mmor less and having a straightness of 1% or less relative to the bladelength.

According to the present invention, the blade portion and the shroudportions preferably are prepared using a ceramic which has an initialJIS four-point flexural strength of 600 MPa or more, a fatigue parameter(N) of 50 or more at a temperature of 1400° C., and a JIS four-pointflexural strength of 50% or more relative to the initial JIS four-pointflexural strength, and a heat-resistant impact strength of 70% or morerelative to an initial heat-resistant impact strength after the bladeportion and the shroud portions are maintained at 1400° C. for 1000hours.

Furthermore, according to another aspect of the present invention, thereis provided a method for preparing a ceramic gas-turbine nozzlecomprising molding compacts of shroud portions and at least one compactof blade portion separately; joining said compacts of shroud portionsand said compact of blade portion to obtain a joined compact byisotropic press molding; calcining said joined compact to obtain acalcined compact; working predetermined positions of said calcinedcompact by a supersonic processing to bore fine cooling holes in saidcalcined compact; and then firing said calcined compact.

The method according to the present invention is capable of preparing aceramic gas-turbine nozzle with fine cooling holes which has alarge-sized and complex shape, their dimensions, structure and the likehaving been previously limited because ceramic's were made intolarge-sized parts, and has a more improved precision than the earlierone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view which shows one example of a ceramicgas-turbine nozzle with fine cooling holes according to the presentinvention.

FIG. 2 is a schematic sectional view which shows one example of a bladeportion of a ceramic gas-turbine nozzle with fine cooling holesaccording to the present invention.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The present invention is illustrated with reference to the followingexamples shown in the FIGS., but the invention is not intended to belimited only to these following examples.

In a ceramic gas-turbine nozzle with fine cooling holes by the method ofthe present invention, one or more blade portions are placed between theinside shroud portion and the outside shroud portion. As shown in FIGS.1 and 2, the interior of the respective blade portion 1 occupies thehollow cavity 3 sandwiched between both of the shroud portions 2 whichare placed in the opposite sides. The interior of this hollow cavity 3communicates with the fine cooling holes 4 and fine cooling holes 5which are formed almost parallel to the shroud portion 2. The finecooling holes 4 are bored into the end of the blade portion 1 which isformed smoothly. On the other hand, the fine cooling holes 5 are boredinto the opposite end of the blade portion 1 which is formed slenderly.Providing the ceramic gas-turbine nozzle with both of the fine coolingholes 4 and the fine cooling holes 5 is preferable to cool the blade,but even providing either of them can preferably cool the same. One orboth of the shroud portions 2 has(have) throughholes 6 which communicatethe hollow cavity 3 to the external space of the shroud portion 2.

The fine cooling holes 4 and 5 are bored by supersonic processing.Though not limited, the sectional shapes of which are preferred to be inthe form of a circle, slit or the like. Further, the fine hole may belinear or curved. Although the deepness of the fine hole is preferred tobe in the range from 20 mm to 70 mm, the ceramic gas-turbine nozzle withfine holes prepared by the method of the present invention is notlimited to the range. The size and shape-precision of the ceramicgas-turbine nozzle with fine cooling holes prepared by the method of thepresent invention, particularly, the shape-precision of fine holes willbe described below.

A refrigerant carrier is introduced from the throughhole 6, passedthrough the hollow cavity 3, and discharged from the fine cooling holes4 and/or 5, so that the outer surface of the ceramic gas-turbine nozzleis cooled. Either gases or liquids can be used as the refrigerantcarrier, but air or water is preferably used in terms of the cost. Ifthe ceramic gas-turbine nozzle is exposed with a combustion gas, byintroducing, for example, the cooling air of a temperature of 400° C. asa refrigerant carrier from the throughhole 6, the temperature of thecooling air becomes 800°-900° C. at the outlet of the fine cooling holes5, and at the same time the surface temperature of the ceramicgas-turbine nozzle becomes 1100°-1400° C. When these cooling conditionsare compared with cooling of the metal gas turbine, decreased heatefficiency of the ceramic gas-turbine nozzle is low because a smalleramount of the refrigerant carrier can be used.

Preferred ceramic gas-turbine nozzles with fine cooling holes accordingto the present invention have a larger heat resistance and decreaseddeterioration in the mechanical strength even at a higher temperature,such as silicon nitride, silicon carbide, stabilized zirconia, partiallystabilized zirconia and alumina.

Examples of the ceramics used in the present invention which havepreferred physical properties include a ceramic which has an initial JISfour-point flexural strength of 600 MPa or more, a fatigue parameter (N)of 50 or more at a temperature of 1400° C., and a JIS four-pointflexural strength of 50% or more relative to the initial JIS four-pointflexural strength and a heat-resistant impact strength of 70% or more,or the other materials' physical properties of 80% or more relative tothe initial values.

The method according to the present invention for preparing the ceramicgas-turbine nozzle with fine cooling holes will be described. For briefdescription, a ceramic gas-turbine nozzle with fine cooling holes inwhich two shroud portions 2 and one blade portion 1 are joined will bedescribed. The present invention is not limited to the ceramicgas-turbine nozzle with fine cooling holes having such a simple shape,and so the ceramic gas-turbine nozzle with fine holes having complexshapes with plural blade portions 1 is also prepared by the methodaccording to the present invention.

By the method of the present invention, two shroud portions 2 and oneblade portion 1 are separately molded, and then the three compacts areintegrally joined. After the joined portions are calcined and thecalcined portions are subjected to supersonic processing to form finecooling holes, the resulting portions are fired and mechanicallyprocessed so as to prepare the ceramic gas-turbine nozzle with finecooling holes.

First, after a sintering auxiliary is added to a ceramic raw materialsuitable for the desired ceramic and mixed, binder(s) and the like areadded to it and kneaded to obtain the raw material for molding.

Secondly, two shroud portion compacts and one blade portion compact asshown in FIG. 1 are molded using the raw material by preferable moldingmethod(s) such as injection molding, press molding, and slip casting.The hollow cavity 3 and the shroud throughhole 6 are formed during themolding processes. All compacts are not necessarily molded by theidentical method, but may be molded by a combination of appropriatemethods. However, when these portions are molded, shrinkage upon joiningthe portions by isotropic press molding in the next process should beconsidered. In this process, the shape and size of the portions can beselected so that deformation of these portions will be minimized aftersintering. Then organic matter is eliminated from these compacts bysuitable methods, depending on molding, such as degreasing andcalcination to remove binder.

After the joint surfaces of the resulting compacts are processed, bothof the compacts are integrally joined by isotropic press molding. Sincethe shroud portion compacts and the blade portion compact which wereseparately molded are then integrally molded according to the presentinvention, a complex-shaped ceramic gas-turbine nozzle with fine coolingholes in which undercuts will be made can be easily prepared. Individualmolding of the shroud portions and the blade portion 1 makes the size ofeach compact smaller compared with molding the gas turbine nozzle as awhole, so that these portions can be easily molded as well as thecompacts having more close shape-precision. The integrated compacts arecalcined at temperatures and under atmosphere depending on the kind ofceramic used to obtain calcined compacts.

These calcined compacts are subjected to supersonic processing at thepredetermined positions to form the fine cooling holes 4 and/or 5. Sincesupersonic processing is subjected to the calcined compact rather thanthe fired compact, the processing time is largely shortened as well asthe processing resistance is decreased, so that shape-precisions such asroundness and straightness and the like of the bored fine holes areimproved (Table 1). Particularly, when fine holes having a diameter of 1mm or less are to be formed, calcined compacts are preferably subjectedto the supersonic processing.

                  TABLE 1                                                         ______________________________________                                               methods according to                                                          the   the     the     the   the   the                                         present                                                                             prior   present prior present                                                                             prior                                       inven.                                                                              art     inven.  art   inven.                                                                              art                                  blade    profile degree                                                                            roundness   straightness                                 length (mm)                                                                            (mm)        (mm)        (mm)                                         ______________________________________                                         50      0.1     1       0.05  0.5   0.1   1                                  150      0.2     2       0.05  0.5   0.3   3                                  ______________________________________                                    

When a subject to be processed is subjected to the supersonicprocessing, a tool applicable to the desired shape is first produced,and the resulting tool is fit on the top of a hone. After the subject iscoated with granulated whetstone, the hone is vibrated up and down togrind the subject by the granulated whetstone present between the tooland the subject.

Then, the subject is fired at firing temperatures and under a firingatmosphere depending on the kind of ceramic used to obtain sinteredcompact. When the sintered compact of the shroud portion 2 is ground,the fitting position between the metal portion and the shroud portion ispreferably subjected to cylindrical or planar grinding using apredetermined diamond whetstone. The fitting position required to have ahigh precision is preferably subjected to the grinding processes. It isnot necessary to subject the other positions to the grinding processesafter sintering.

Then, Examples are hereinafter illustrated in more detail.

EXAMPLE 1

Two parts by weight of strontium oxide, 3 parts by weight of magnesiumoxide and 3 parts by weight of cerium oxide as sintering auxiliaries areblended to 100 parts by weight of silicon nitride powder, then ground toobtain a mixture of powder having an average particle size of 0.5 μm.The thus obtained mixture was spray-dried to obtain a granule having anaverage particle size of 30 μm.

Using the granule which was obtained by spray drying as a row materialfor the press molding, two compacts of the shroud portions shown in FIG.1 were obtained by subjecting the granule itself to a molding pressureof 0.5 ton/cm². The surface of the shroud portion compact was finishedby machining the joint interface of the compact.

On the other hand, when a raw material for the injection molding isprepared, 3 parts by weight of ethylene-vinyl acetate copolymer as anorganic binder, 15 parts by weight of paraffin wax as a plasticizer, and2 parts by weight of stearic acid as a plasticizer were blended to 100parts by weight of the granule obtained by spray drying to knead. Themixture was extruded by a extruder to be in the form of a pellet, whichwas used to mold the blade portion compact having the shape shown inFIG. 1 by injection molding.

After the blade portion compact was degreased at 300° to 500 ° C., thesurface of the blade portion compact was finished by machining the jointinterface of the compact in the similar manner to machining the shroudportion compacts obtained by press molding.

The above-mentioned blade portion compact and two shroud portioncompacts were combined at the joint surface, and integrally joined bycold isotropic press molding under a pressure of 3 to 5 ton/cm² toobtain the joined compact as shown in FIG. 1.

The joined compact was calcined at 1000° C. for 30 minutes, and then thecalcined compact was subjected to the supersonic processing to bore finecooling holes. The resulting calcined compact was fired at 1700° C. innitrogen atmosphere to obtain a fired body. Finally, the area of shroudportions 2 of the fired compact which attaches to other parts was groundwith a diamond hone to obtain the ceramic gas-turbine nozzle with finecooling holes according to the present invention.

EXAMPLE 2

The ceramic gas-turbine nozzle with fine cooling holes was prepared inthe similar manner to that in Example 1 except that the two shroudportions were molded by the injection molding rather than the two-steppress molding.

Evaluation

The ceramic gas-turbine nozzle With fine cooling holes obtained in theabove-mentioned Example 1 was tested for the evaluation of the followingphysical properties.

The ceramic gas-turbine nozzles with fine cooling holes shown in FIG. 1,which was obtained in Example 1 and 2, was maintained in a flowingcombustion gas at 1500° C. for 100 hours, while cooling air at 300° to400° C. whose volume was 1 to 5% relative to the total compressed airflowed through the fine cooling holes. As a result, both of the ceramicgas-turbine nozzles with fine cooling holes have an average surfacetemperature of 1200° C.

When flowing of the combustion gas was stopped to observe the ceramicgas-turbine nozzle after the test, there were no changes in any nozzles.Further, the test pieces which were excised from the nozzle after thetest were subjected to tests such as a four-point flexural strengthtest, showing nearly the same values as those of the initial strength,and thus there were no changes in the strength.

What is claimed is:
 1. A method for preparing a ceramic gas-turbinenozzle comprising:separately molding a plurality of shroud portioncompacts and at least one blade portion compact; joining said compactsby isotropic press molding to form a joined compact; calcining saidjoined compact to form a calcined compact; working predeterminedpositions of said calcined compact by a supersonic processing to borefine cooling holes in said calcined compact; and firing said calcinedcompact.